Thanks to all the recent x86 entry code refactoring, most tasks' kernelstacks start at the same offset right above their saved pt_regs,regardless of which syscall was used to enter the kernel. That createsa nice convention which makes it straightforward to identify the"bottom" of the stack, which can be useful for stack walking code whichneeds to verify the stack is sane.

However there are still a few types of tasks which don't yet follow thatconvention:

1) CPU idle tasks, aka the "swapper" tasks

2) freshly forked TIF_FORK tasks which don't have a stack at all

Make the idle tasks conform to the new stack bottom convention bystarting their stack at a sizeof(pt_regs) offset from the end of thestack page.